This invention relates to an electrolyte-cathode assembly for a solid electrolyte fuel cell suitable for operation at high temperatures, of the type comprising a gas-tight body made of solid ceramic electrolyte capable of conducting oxygen ions and at least one layer of electronconducting material in contact with the electrolyte and secured thereto.
It is known that in a certain kind of solid-electrolyte fuel cell operating at a temperature generally of the order of 600.degree. to 1000.degree. C., the electrolyte is an oxide having cavities in its structure, for example stabilized zirconium dioxide, having a relatively high oxygen-ion conductivity in the aforementioned range of operating temperatures.
In a fuel cell of this kind, the cathode usually comprises a layer of a metal, for example silver, which is a good conductor of electricity and which does not oxidize under the cell operating conditions, the metal layer being in contact with the electrolyte and secured thereto.
Known metal cathodes are not completely satisfactory since they have the disadvantage of becoming detached, at least partly, from the electrolyte surface after a varying period of use, thus reducing their active surface. In addition, metal cathodes may be subject to recrystallization, which modifies their porous structure and adversely influences their electrochemical properties and their lateral electric conductivity.
A considerable research effort has been made in order to find and develop cathodes having low polarization combined with a long life.
Swiss patent specification No. 478 463 describes an electrode which can be used either as the cathode or as the anode of a solid electrolyte fuel cell operating at high temperature. The electrode comprises a first porous layer made of granules of a ceramic material conducting oxygen ions and electrons, the first layer being in contact with the electrolyte and secured thereto, and a second porous layer covering the first layer and comprising a material having a high electron conductivity, for example a metal or a ceramic material.
Swiss patent specification No. 540 571 describes an electrolyte-electrode assembly comprising a sealed-tight body made of a solid electrolyte which conducts oxygen ions at high temperature and an electrode made up of at least one layer of doped indium oxide and covering part of the surface of the electrolyte and adhering thereto, the layer of doped indium oxide being made up dendrites the major axes of which are perpendicular to the electrolyte surface.
An object of the present invention as exemplified hereafter is to provide an electrolyte-cathode assembly in which the cathode has polarization characteristics at least as good as those of the electrodes described in the aforementioned Swiss Patent Specifications, but which also has an even better service life than the aformentioned electrodes.